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# Joist load question, travertine 18"

#1
09-17-07, 08:52 AM
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Join Date: Sep 2005
Location: USA
Posts: 192

I found an online deflection calculator that calculates the maximum unsupported span based on the specs of your joists. Will this be sufficient for figuring out if I need to sister joists in my floor?

http://www.awc.org/calculators/span/...rcalcstyle.asp

If I have to buy and sister joists to complete this project it's likely to stop before it began.

I am tiling my kitchen. My house was built in the northeast in 1994, from what I understand we have pretty good
Facts:
1. floor joists 16" o.c.
2. 3/4" ply subfloor
3. 10.5' unsupported span
4. joists are dimensional lumber (checking height tonight)
5. Travertine is 18"x18"x1/2"

Questions
Will this be good with a 1/2" hardibacker? and 1/2" trowel for the thinset?
Does the hardibacker have to be glued to the plywood?

#2
09-17-07, 11:07 AM
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Join Date: Sep 2005
Location: USA
Posts: 192
Ok, from what I've found online it doesn't appear that I'm going to be able to get around atleast adding one layer of ply. True?

So does this sound like enough:
Add this to existing 3/4" subfloor
1. 1/2" t&g ply or osb
2. thinset for hardi backer
3. hardi backer 1/4"
4. thinset for tiles 1/2" tooth
5. tiles

Will that be sufficient?

#3
09-17-07, 12:34 PM
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Join Date: Jan 2006
Location: USA
Posts: 3,505
Use this calculator to calculate deflection for your travertine installation

http://www.johnbridge.com/vbulletin/deflecto.pl

Provided that you meet L720 deflection your plan sounds good.

1. Use 1/2" bc grade exterior plywood over your existing 3/4" plywood subfloor, not osb.

2. Yes thinset under the hardi 1/4" notched trowel

3. Yes 1/4" hardi is all you need.

4. You can try 1/4" x 3/8" notch trowel first and see if you are getting adequate coverage. You'll probably have to go to 1/2" notch for that size travertine. Id use a medium bed mortar for this.

Glad to see youve done some homework before you start.

#4
09-17-07, 01:10 PM
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Join Date: Sep 2005
Location: USA
Posts: 192
Thank you so much for that info. It was very valuable, couple more questions.

1. I need the thinset to attach the hardi to the plywood underlayment. Do I also need screws?

2. What kind of screws for the underlayment plywood to the existing subfloor? I keep seeing "not dywall screws" does that mean not course thread but fine thread?

3. Is the 1/2" bc grade exterior plywood a bigbox buy or local lumber yard? Is this it?
http://contractorservices.homedepot....a-d49e58036906
I hope not because that sounds like another \$200 tacked on to this project.

#5
09-17-07, 01:18 PM
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Join Date: May 2007
Location: USA
Posts: 1,607
I agree with Johnny. Only thing different is that I say you have to use medium bed thinset mortar and has to be mixed right. There are many medium bed mortars available. Let us know what brands you can get if you need a specific recommendation. The trowel size depends on the floor and your technique. I would have several choices handy so that you will get the proper transfer of mortar.

Jaz

#6
09-17-07, 01:33 PM
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Join Date: May 2007
Location: USA
Posts: 1,607
Absolutely, you have to fasten! You can use the special backerboard screws OR roofing nails. If you choose the special screws, be sure they are the square-drive type NOT Phillips head, which stink. NO fine thread, fine thread is for hardwood which of course would be wrong. I always used to use roofing nails because I have a roofing gun and found it much faster and the heads usually set flush. Many people have had trouble getting certain screws to seat properly. I said 'used to' because I would only use Ditra now for most jobs.

Your plywood link didn't work for me but it's probably in the \$20 range? You can use C/C plugged if you can find it. Just make sure it's "underlayment" grade, NOT "sheathing".

Jaz

#7
09-17-07, 01:40 PM
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Join Date: Jan 2006
Location: USA
Posts: 3,505
Yes Jaz I agree I should have said "have to use medium bed mortar".

Yes you have to screw or nail the hardi to the plywood. You do this immediately, as soon as you set the hardi in thinset. There are special backer board screws (square drive heads) that you'll find in the same isle as the hardi at the big box stores. You can also use galvanized roofing nails. Go to the hardi website and read their installation instructions.

Use deck screws to screw the 1/2" ply to the existing subfloor - 1 1/2" screws should be good. Use lots of screws, dont be bashful. Every 6" around the plywood edges and every 8" in the field. Make sure the plywood edges dont line up with the plywood subfloor below, and avoid screwing into the joists as best as possible - just screw plywood to plywood. Leave 1/8" expansion gap between plywood sheets and leave about 1/4" around the room.

Your link to HD just goes to their homepage, but yeah, bc plywood is expensive stuff. You need to use the bc or better because anything less has voids that could cause the installation to fail.

Boy that Jaz types fast. Guess I need to learn to type with more than 2 fingers.

Last edited by HeresJohnny; 09-17-07 at 01:58 PM. Reason: Jaz beat me to it.

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